The leaching of inorganic species from activated carbons produced from waste tyre rubber
Introduction
Around 400,000 t/yr of used tyres are generated in the United Kingdom [1] and over 2,500,000 t/yr in the United States [2]. Owing to their flexible nature and resistance to degradation, landfilled [3] whole tyres occupy large volumes and have been reported to cause the destabilisation of compacted landfill sites [1], [2], [4]. Scrap tyres also represent a significant fire hazard. Fires in tyre deposits are extremely difficult to control and have been reported to generate high levels of pollution to the atmosphere, soil, surface waters and ground waters [1], [4], [5], [6].
Pyrolysis is a recycling alternative which involves the heating of tyres under inert conditions, usually at temperatures between 500–700°C. The process generates a carbonised char, which consists primarily of carbon black and carbonised rubber polymer, and a volatile fraction which can be separated into a condensable hydrocarbon oil and a high calorific value gas [7], [8], [9], [10], [11].
A number of publications have discussed the controlled gasification of rubber-derived pyrolytic chars using steam or carbon dioxide for the production of high quality activated carbons [12], [13], [14], [15], [16], [17], [18], [19]. These carbons, which are produced in a powdered form, have been reported to exhibit surface areas, porosity and adsorption characteristics comparable to commercial adsorbents employed in drinking and waste water treatment applications.
However, the rubber feed contains high levels of inorganic impurities, particularly zinc and sulphur, which are used as additives in the rubber production process [20]. There are concerns that the leaching of some of these species may restrict the use of the rubber-derived carbons in liquid phase applications, particularly those which require high purity standards such as the treatment of drinking water.
This work constitutes the first study on the leaching of inorganic impurities from activated carbons produced from waste tyre rubber. For the purpose of this work, a number of activated carbons were produced using a laboratory-scale rotary furnace. The carbons were characterised for their BET surface area and inorganic composition and results were compared with those obtained from two commercial activated carbons employed in water treatment applications. Selected carbon samples were subsequently investigated for the leaching of inorganic impurities into solution at different pH values and carbon doses.
Section snippets
Production of tyre rubber-derived carbons
The tyre rubber employed in this work was supplied by Duralay Ltd. (UK) in a powdered form of particle size <0.42 mm. Two widely used commercial powdered activated carbons, bituminous coal-based Chemviron GW (Chemviron Carbon UK Ltd.) and lignite based Hydrodarco-C (Norit UK Ltd.), were use for comparative purposes.
A Carbolite HTR 11/150 laboratory-scale rotary furnace, described in Ref. [10], was used to produce the rubber-derived carbons. During the production of each sample, the furnace was
Carbon yields, surface area and ash contents
As described in Table 1, the pyrolysis of tyre rubber generated a carbonised char which represented 42 wt% of the initial rubber mass. The volatile fraction was removed from the reaction vessel as it was produced and condensed in a glass trap at room temperature. The transit of the volatile fraction was facilitated by the continuous purge gas flow, which reduced the residence time of the vapours inside the furnace. The amount of condensable hydrocarbon oil recovered represented 52 wt% of the
Conclusions
Tyre rubber has proven to be an excellent precursor for the production of large surface area activated carbons. However, the tyre rubber contains large concentrations of zinc (12,700 ppm) and sulphur (16,200 ppm), as well as traces of other potentially harmful metals such as lead (59 ppm), cadmium (2.9 ppm), chromium (49 ppm) and molybdenum (10 ppm), which may concern potential users of these carbons.
Results from this work show that all inorganic species present in the rubber are concentrated in the
Acknowledgements
One of the authors (Guillermo San Miguel) would like to thank the Departamento de Educación, Universidades e Investigación of the Basque Regional Government (Spain) for financial support.
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